Mirrors for various uses are known in the art. For example, see U.S. Pat. Nos. 5,923,464 and 4,309,075 (all hereby incorporated herein by reference). Mirrors are also known for use in projection televisions and other suitable applications. In the projection television context, see for example U.S. Pat. Nos. 6,275,272, 5,669,681 and 5,896,236 (all hereby incorporated herein by reference).
One type of mirror is a second or back surface mirror (most common), while another type of mirror is a first or front surface mirror (less common). Back surface mirrors typically include a glass substrate with a reflective coating on a back surface thereof (i.e., not on the front surface which is first hit by incoming light). Incoming light passes through the glass substrate before being reflected by the coating in a second surface mirror. Thus, reflected light passes through the glass substrate twice in back or second surface mirrors; once before being reflected and again after being reflected on its way to a viewer. Mirrors such as bathroom mirrors, bedroom mirrors, and architectural mirrors are typically back or second surface mirrors so that the glass substrate can be used to protect the reflective coating provided on the rear surface thereof.
In applications where more accurate reflections are desired, front (or first) surface mirrors are sometimes used. In front/first surface mirrors, a reflective coating is provided on the front surface of the glass substrate so that incoming light is reflected by the coating before it passes through the glass substrate. Since the light to be reflected does not have to pass through the glass substrate in first surface mirrors (in contrast to rear surface mirrors), first surface mirrors generally have higher reflectance than do rear surface mirrors, and no double reflected image. Example front surface mirrors (or first surface mirrors) are disclosed in U.S. Pat. Nos. 5,923,464 and 4,780,372 (both incorporated herein by reference).
Unfortunately, glass substrates that support mirror coatings (or other coatings such as low-E coatings) sometimes have surface non-uniformities defined therein due to the process of manufacturing the glass (e.g., float glass manufacturing process). For example, glass made via the float process often experiences so-called “draw lines” in the glass which cause a surface of the glass to realize a number of spaced apart hills/valleys or undulations. These hills/valleys (or undulations) caused by the draw lines in the glass cause a major surface(s) of the glass to realize a type of waviness so that it is not perfectly flat.
Draw lines lead to problems because coatings (e.g., mirror coatings or low-E coatings) formed on glass having draw lines tend to be conformal to the glass surface. Thus, the coatings on the glass also realize the hills/valleys due to their conformal nature. Unfortunately, this can lead to distortions of images in the case of reflected images from mirrors (first surface or second surface). In particular, due to the hills/valleys in the coating that are caused by the draw lines in the underlying glass substrate, some of the reflected light may be directed in a direction which deviates from an expected direction of reflection. This can lead to problems with reflected images from first or second surface mirrors, distortions of TV images in the case of first surface mirrors used in projection TVs, and/or non-uniform reflection characteristics in the case of mirror or low-E coatings.
It will be apparent from the above that there exists a need in the art for a technique for reducing adverse affects caused by non-uniformities such as draw lines in glass in the context of coated articles such as mirrors, windshields, and/or the like.